All electrical power converters delivering direct-current power to a load, and deriving this power from an alternating-current source, must process it through at least one stage performing a rectification function. Output circuits comprise secondaries, and secondaries comprise secondary circuits. In the case of switch mode (SM) power conversion with direct-current galvanically isolated secondaries there are typically a minimum of two stages where rectification occurs, the second of such being in the secondary circuit. All direct-current to direct-current SM power converters with direct-current galvanically isolated secondaries have at least one stage where rectification occurs, located in the secondary circuit.
Greater power conversion efficiency results in the secondary circuit from having greater output load voltages, where the increases to the value of output load voltage has the greatest incremental influence on increases to efficiency when the ratio of that load voltage magnitude to the combined voltage magnitudes of forward voltage drops of the secondary rectifier(s) and stray impedance(s) is smallest, this being true due to the same current conducting through the forward voltage drop of the output load also conducting through the power dissipative forward voltage drops of the rectifier(s) and stray impedance(s).
The terms rectification, rectifier, or rectifier(s), as used herein shall also be construed to signify and comprise any means of rectifier optimization such as synchronous rectification that may include but not be limited to diodes, MOSFETs, MOSFETs and diodes, active switching devices, active switching devices and diodes.
There is increasing market emphasis on producing or procuring solutions for power conversion that are optimized to be efficient, delivering a high percentage of the electrical power they consume from their supplies into electrical power available to their loads. Emphasis on efficiency is due in some applications to market emphasis on optimizing for low total cost of ownership (TCO) of an electrical system, especially where a favorable TCO result is necessary to justify a high initial expenditure for purchase of a system, or in an industry where lowering operating cost is essential to maintaining or improving profitability, cost-competitiveness, or market viability. There is also increasing emphasis on so-called energy efficiency as a national priority in the United States and other countries that are net energy importers. Energy-efficiency encompasses power conversion efficiency, but above all signifies prioritizing efficaciousness of energy utilization.
An electrical system may comprise power conversion apparatus, cooling apparatus, fixture and housing apparatus, and an output electrical load apparatus. One example of an electrical system where market emphasis is on TCO is that of a luminaire utilizing power LEDs for light generation, where a favorable TCO benefit analysis is often pivotal to a decision by market participants to engage said LED technology through the adoption of system embodiment that successfully preserves at the system level the sought benefit of an efficacious light source that the LEDs embody at their elemental level. The electrical system of the example luminaire should be capable of supporting a favorable TCO analysis by offering low operating costs through having its power conversion efficiency optimized to a suitable level, while limiting the cost of its manufacture through inherently less costly cooling apparatus enabled by higher efficiency, and inherently less costly housing apparatus afforded by a lower maximum secondary working voltage magnitudes.
For purposes of illustration to demonstrate the effect upon efficiency of the secondary rectifier(s) in an power conversion apparatus, the LED load voltage in a typical luminaire application due to a load comprising a single LED, or several LEDs receiving their power in parallel connection, is assumed to be approximately 3 volts, where an additional forward voltage drop of each current conducting rectifier series electrically connected between transformer output and output load in the secondary circuit may be assumed to be between 0.3 volts and 1.2 volts, and where other additional forward voltage drops due to stray and other impedances also series electrically connected in the secondary circuit between transformer output and output LED load may be assumed to be insignificant by comparison.
In the case of the example electrical system, an output load may be redesigned so it is optimized to support greater load voltage, either by reconfiguring its LED load to have its several parallel LEDs electrically connected instead in series, or reconfiguring its load comprising only a single LED by replacing it with multiple lower luminosity LEDs electrically connected in series so that they have equivalent total luminosity as the single LED being replaced. It should be assumed for the sake of simplicity with this first illustration that output power remains constant as a function of overall luminosity remaining constant, so LED load current will be decreased as a result of the LED load voltage having been increased for this comparison at constant luminosity between original configuration and reconfigured loads, given that output power is identical for both cases, where power is calculated as the product of load current and load voltage. Higher load voltage will result in higher electrical efficiency than lower load voltage for a constant power load, due to relative reduction in electrical power being dissipated by a decreased load current conducting through the forward drop of the rectifier(s) series electrically connected in the secondary circuit between transformer output and output LED load, compared with the unchanged electrical power continuing to be delivered to the higher voltage of the reconfigured LED output load.
One may analyze a second case using this example, where the load current remains unchanged and output load voltage as well as output power is increased. For any given load current in this second case, efficiency is increased due to the increased output load voltage magnitude, and its consequence of a now increased proportion of power delivered to the output load, relative to an unchanged amount of power dissipation due to current conducting through the forward drop of the secondary rectifier(s).
To increase efficiency by increasing output load voltage using a current-compliant voltage source (CCVS) class of topologies would entail a transformer that behaves as a CCVS, which would result in working voltage magnitudes in secondary circuit(s) significantly exceeding the voltage magnitude of the output load, due to the voltage compliant behavior of inductor(s) series electrically connected in secondary circuit(s) conducting current for use by the output load(s), said voltage magnitudes possibly increasing the risk of electric shock to operators who come into physical contact with bare conductors in said secondary circuit(s), or increasing risk of fire or explosion in certain environments. To meet a need for safety in the case of operators coming into physical contact with electrically conductive elements, the power industry has adopted a discipline of redundancy, or equivalent redundancy, regarding electrical insulation systems and maximum permissible working voltage magnitudes; and required that in those situations that safety be maintained despite any single failure occurring. In the case of protecting against fire or explosion in certain high risk environments, criteria are established to maintain working voltage magnitudes below permissible maximum limits despite the simultaneous occurrence of any two failures. It can be anticipated that other cases exist that require criteria be met for an even higher degree of redundancy protection, such as in so-called mission critical applications.
An operator may be afforded safety protection from electric shock due to physical contact with voltage present on bare conductors of the output load if the secondary circuit ensures that under normal conditions, or due to a single fault, the maximum voltage level considered to be safe shall not be exceeded on the bare conductor of the output load. To address this need a voltage monitor circuit for over-voltage fault detection may be employed that acts to ensure no over-voltage condition exists at the secondary's output terminals due to any single fault, and in addition the secondary circuit must be constructed to insulate against electric shock by some inherent means such as a grounded or insulated housing protecting the operator from exposure to bare conductors of the secondary circuit exclusive of its output terminals. In the example of a luminaire, where for instance it may be desired to allow an operator to simply screw-in a replacement as for incandescent bulbs by using LED luminaire systems with similar form and function to that of an incandescent bulb, this requirement for a housing of secondary circuits may pose a difficulty assuming no grounding point is available, such as is the case with a traditional incandescent light bulb for instance, and that an appropriate insulating enclosure protecting bare conductors in the secondary circuit from operator contact would be difficult to implement while retaining TCO benefits to luminaire of efficacious LED technology due to reduced housing volume.
A requirement therefore that is often imposed on any successful solution to an application such as illustrated by the luminaire example is that of electrical safety for the operator. Electrical safety is a function of the human body's tolerance to applied voltage, reduction to the risk that an operator may come into physical contact with hazardous voltages, and limiting the maximum magnitudes for the two types of voltages that an operator may come into contact with, direct-current or alternate-current. Safety related to electrical circuits is also a function of addressing circumstances of certain environmental conditions which may allow for increased risk of fire or explosion, and mitigating against these effects by ensuring voltages do not exceed certain thresholds despite the possibility of multiple simultaneous faults occurring. Criteria that address electrical safety provisions for providing protection from electrical shock are found in several industry-accepted electrical standards such as described in the cited reference document: Bob Mammano, Lal Bahra, “Safety Considerations in Power Supply Design”, Texas Instruments. Criteria that address electrical related safety provisions for providing protection from risk of igniting a fire or explosion are found in several industry-accepted electrical standards such as described in the cited reference document: “AN9003-9 A Users Guide To Intrinsic Safety”, October 2006, MTL Instruments, Power Court, Luton, Bedfordshire, England. A circuit may qualify for the classification of ‘Extra Low Voltage’ (ELV) if it protects against the risk of electric shock due to hazardous voltages in the event an operator comes into physical contact with its bare conductors. If protection continues to be effective in the event of any single fault, the circuit may qualify for classification of ‘Safety Extra Low Voltage’ (SELV). If protection continues to be effective, as defined for hazardous environments, in the event of any two simultaneous faults, the circuit may qualify for classification of ‘Intrinsically Safe’ (IS).
The subject circuit being considered for its electrical safety or related environmental safety may be an output load, the output terminals of a secondary circuit, or the secondary circuit. Any circuit connected to another may be able to share the same benefit of safety voltage class under usual circumstances. An output load is at a minimum a secondary circuit if it is connected only to the output terminals of a secondary circuit. If said secondary circuit's output terminals are classified SELV due to limiting its maximum voltage magnitude under both normal or fault conditions, and the output load is incapable of producing higher voltage magnitudes than it is supplied by the secondary circuit, then the output load may also be classified SELV. Output terminals of a secondary circuit may be classified SELV without requiring its secondary circuit be so classified, if no non-SELV bare conductors are external to the secondary circuit enclosure, and if non-SELV bare conductors internal to the secondary circuit are inaccessible to an operator; and if an over-voltage fault detection and fault limit circuit is employed to monitor voltage at said accessible terminals that acts sufficiently fast in event of a fault to prevent from appearing on said terminals any higher voltage magnitude than intended within the criteria for classification of SELV circuits. In this case it is not necessary for a secondary circuit to be rated SELV, in order for the load to be SELV. A common practice used with non-SELV secondary circuits to ensure electrical safety is to employ a grounded conductive housing enclosure, meeting established material and structural requirements, suitable for making bare conductors in the secondary exclusive of its output terminals inaccessible to an operator. Satisfying an isolation requirement of a non-SELV secondary circuit in order to have its output terminals and output load rated SELV imposes additional manufacturing costs and physical space requirements upon the system design, than would be the case where the secondary circuit is classified SELV. An advantage for an electrical system due to having secondary circuits rated SELV would therefore result in more favorable TCO analysis, in cases where it is a requirement that the load must be SELV. A further advantage for an electrical system, which is due to having higher secondary circuit efficiency, would result in less costly cooling apparatus, fixturing apparatus and housing apparatus, therefore lower cost to manufacture electrical system with subsequently more favorable TCO analysis being made possible.
Transformers are useful for meeting a requirement of secondary circuits to be direct-current galvanically isolated from hazardous working voltages on primary conductors. Standards described herein define specific classes of insulation as being ‘functional’, ‘basic’, ‘supplemental’, ‘double’, or ‘reinforced’; and further describe substituting a grounded enclosure as an option in lieu of a transformer's supplemental insulation. The transformer construction may employ insulation that is classified as ‘functional insulation’. ‘Functional insulation’ does not provide protection against electric shock, or provide safety, it being assumed to possibly have small defects. ‘Basic insulation’ does provide basic protection against electric shock, but does not provide safety. ‘Supplementary insulation’ also provide basic protection against electric shock, but does not provide safety, however may be used together with basic insulation so that the insulation system comprising both basic and supplementary insulation is classified as ‘double insulation’, which does provide electrical safety protection under normal conditions or in the event of any single fault. ‘Reinforced insulation’ is a single-insulation system equivalent to double insulation in providing electrical safety protection. Other requirements for ELV and SELV classification such as minimum distances for creepage and clearance, insulation temperature class and maximum allowed hot spot temperature, and more, are further specified in the criteria herein described in the cited reference document: Bob Mammano, Lal Bahra, “safety Consideration in Power supply design”, Texas Instrumants. For a secondary circuit to be classified as SELV when it is magnetically coupled through any transformer(s) to a primary circuit, besides having said maximum limits on its secondary voltage magnitudes it is required that the transformer insulation system be either ‘double’ or ‘reinforced’ type. It is possible to substitute a protective earth shield, comprising a conductive grounded enclosure, in place of ‘supplementary’ insulation and, together with ‘basic’ insulation, satisfy SELV criteria for an insulation system. Circuits so classified ELV or SELV must not have working voltage magnitudes that exceed limits given in relevant standards: for example, 42.4 Volts alternating-current; or 60 Volts direct-current. One advantage of having a circuit classified SELV instead of ELV is that an operator may be permitted safe unrestricted access to its bare circuit components.
Another classification that describes limited exposure levels for voltages in order to address electrical safety hazard concerns is that of ‘Telecommunications Network Voltage’ (TNV), where the normal operating voltage magnitudes may not exceed, for example, 71 Volts alternating-current or 120 Volts direct-current, with an additional condition that the operator-accessible contact area is limited only to the surface of an exposed connector pin. Voltage magnitudes may be somewhat greater for limited time durations, as further described in industry-accepted relevant extant standards.
It would represent an advancement to the art if a means were provided to those of ordinary skill in the art for choosing and utilizing an optimal power converter topology, or class of topologies, which allows simultaneous improvement to secondary circuit efficiency and reduction in secondary circuit working voltage magnitudes. In order to proceed with developing a framework within which to explore how working voltages are imposed in secondary circuits, towards a goal of seeking methods to limit those voltage magnitudes reliably in the context of electrical safety, it is essential to understand the difference in behavior between impedances differentiating between those that are voltage-compliant and those that are current-compliant, and to understand how the impedance of a secondary circuit may be compliant with compatible sourcing behavior of the primary circuit from which it receives its power. It is also essential to understand the relationship between output load voltage magnitude and secondary circuit working voltage magnitude levels in order to increase secondary circuit efficiency through higher load voltage magnitude levels, while promoting secondary circuit electrical or related environmental safety through lower working voltage magnitude levels.
It should be understood that power converters are often manufactured and made available for sale to the market without attached loads. To increase generality of this description, the terms ‘load voltage’, ‘output voltage’, and ‘output terminal voltage’ may be used to essentially signify the same differential voltage, said differential voltage appearing at the output terminals of the power converter whether those terminals are real or virtual, and whether the power converter is supplying an external load, an internal load, or is unloaded.
Many SM electrical power converters employ transformers to provide direct-current galvanic isolation between their input, herein called primary in the case of connection to primary circuits connected directly to the mains or other sources having hazardous voltage levels, and their output, herein called secondary; and in addition transformers perform power transformation from one set of current and voltage variables received as input on their primary into another that they produce as output on their secondary. Alternating-current current and alternating-current voltage are received as inputs by the direct-current galvanically isolating transformer due to their generation through the forcing function actions of an primary alternating-current power stage, generally involving electronic power switches that are controlled to be either on, or off, and cycled continuously to produce a periodic voltage waveform. A capacitor may be connected in series between this periodic voltage waveform, which may include a direct-current component, and the input to the primary of the transformer. One purpose of the capacitor is to allow higher frequency components to conduct but to block direct-current components from reaching the primary of the direct-current galvanically isolating transformer. The secondary circuit provides rectification of the alternating-current current that sources from the transformer output, said rectification introducing direct-current components into the rectified current. Additionally, in one class of topologies a filtering function is generally employed using a series inductor and a shunt capacitor connected in the secondary circuit, which results in achieving a low-pass function on the power converter's output voltage. In another class of topologies, only a shunt capacitor may be connected in the secondary circuit for filtering purposes. Where both alternating-current components and a series electrically connected filter inductor are present in the secondary circuit, the working voltage maximum magnitude in the secondary circuit can significantly exceed the voltage magnitude of the secondary circuit's output terminals. This secondary circuit that includes an inductor forms a voltage-compliant load to the primary circuit, and is compliant with a primary circuit that behaves as a current-compliant voltage source (CCVS).
It is understood by those of ordinary skill in the art of isolated power converter design that a direct-current galvanic isolation barrier must exist between primary and secondary circuits of isolation converters, and these barriers must adopt effective isolation means for each interface where signals or power pass between the primary and the secondary, including but not limited to feedback signals, fault signals, and control signals, as appropriate to each converter design.
A voltage-compliant current source (VCCS) can be realized through the behavior of an inductor. Inductor current cannot change instantaneously and inductor differential voltage may vary to be compliant with behavior of the circuit that it connects to. A CCVS can be realized through the behavior of a capacitor. Capacitor differential voltage cannot change instantaneously and the capacitor current may vary to be compliant with behavior of the circuit that it connects to. The characteristic of a variable type not being able to change instantaneously is termed herein as a source of that variable type. The characteristic of a variable type that is able to change instantaneously is termed herein as being compliant of that variable type.
SM power converter circuits can employ a class of VCCS topologies with full wave rectification, a class of VCCS topologies with half wave rectification, or a class of CCVS topologies, where the VCCS and CCVS designations refer to behavior of the primary power circuit acting upon the secondary power circuit. In all topologies, a rectifier is series electrically connected with the output load, and installed before any secondary circuit series inductor or secondary circuit shunt capacitor, and installed after the transformer secondary terminals, for the purpose of rectifying an alternating-current current without direct-current current on its input into a direct-current current with possible alternating-current current components, and allowing the shunt capacitor to subsequently produce a direct-current voltage component on its output with possible alternating-current voltage components.
The more traditional class of topologies involve a primary power circuit that acts as a CCVS, and the rectifier-low-pass-filter circuit combination connected in the secondary circuit comprises series rectifier followed by series inductor and shunt capacitor, which may produce a nearly direct-current voltage waveform at the secondary circuit output due to the averaging function the filter performs, retaining the direct-current component of rectified alternating-current current and attenuating the alternating-current components of filtered output voltage.
A CCVS topology does not have substantial inductance series electrically connected with the primary of the transformer, and instead has a filter inductor series electrically connected with the secondary of the transformer, where it allows the transformer secondary to behave as a CCVS, compliant with the load characteristics of imposed current behavior by the relatively high impedance of the secondary filter inductor. A special category of CCVS topologies is a phase-shift modulated quasi-resonant zero voltage switch (ZVS) topology that, besides employing a secondary circuit connected filter inductor as do other CCVS topologies, has only as much inductance intentionally connected in the primary, either as a discrete element, stray and leakage inductances, or some combination, to provide the small amount of energy, in the form of voltage-compliant current sourcing, required in order to effect so-called soft resonant transition of the switches from one state to another thereby reducing the hard-switching power losses that would otherwise result. The inductance employed in the primary circuits of these quasi-resonant schemes is insignificant in relation to that employed in their secondary circuits, said secondary filter inductance performing a low-pass filtering function in conjunction with a secondary shunt capacitor. In the CCVS class of topologies, including these quasi-resonant ZVS topologies just described, the maximum operational voltage magnitude in the secondary side circuit may be significantly higher than the maximum output load voltage magnitude. Utility of the CCVS class of topologies does allow for higher efficiency through higher output load voltage values, but at a cost of imposing working voltage magnitude values in the secondary circuit that are substantially higher than the output load voltage magnitude, and that at relatively low output voltage levels may result in hazardous voltage levels in the secondary circuit.
In any topology of the VCCS with full wave rectification class an inductor of significant value may be series electrically connected to the transformer primary. A capacitor is also electrically connected in the primary in series with the inductor. In resonant topologies of the VCCS with full wave rectification class the placement of a significant inductance in electrical series connection to the primary of the transformer, in addition to a series electrically connected capacitor and resulting resonant behavior, is motivated in some instances, at least in part, by the desire to capture an advantage of reducing the switching power losses associated with the primary switches that create the alternating-current waveform for the transformer. A rectifier that produces a direct-current component on its output by rectifying an alternating-current waveform on its input is necessary to be series electrically connected with the secondary of the transformer, as direct-current components cannot be passed through the transformer from its primary to its secondary. In this case of the VCCS with full wave rectification class of topologies the inductor series electrically connected in the primary circuit and being excited by a voltage waveform forcing function generated by cyclic on and off actions of the switches, conducts only alternating-current components as ensured by the direct-current blocking behavior of the series electrically connected capacitor. The inductor attenuates higher frequency harmonics to a greater degree than lower frequency harmonics, in so doing acts also as a low pass filter. The primary inductor and its effect as a low pass filter, as well as the primary capacitor and its effect as a high pass filter, together acting as a band-pass filter, appear in equivalent series electrical connection to the output load, and the secondary capacitor may be treated analytically as appearing in equivalent shunt connection to the trsnsformer primary, when the secondary rectifier, secondary capacitor, and output load are reflected analytically from the secondary to the primary side of the transformer. The secondary rectifier will introduce direct-current on its output, or load side, from alternating-current of the band-pass filter on its input, or transformer side. Acting together with the secondary rectifier and secondary shunt capacitor, the inductor on the primary performs a low-pass filtering operation, that produces primarily direct-current on the output terminals.
This primary circuit with its series electrically connected inductance and series electrically connected capacitance in VCCS topologies behaves resonantly when excited by forcing function frequencies near its natural frequency/ies. In the case of an inductor series electrically connected to the primary of the transformer of VCCS topologies, it is desired to allow waveforms at the forcing function switching frequency/ies to pass for the purpose of providing an alternating-current power source to the transformer's input, and at a frequency or range of frequencies for which the transformer was designed; while in the contrasting case of a filter inductor series electrically connected to the secondary of the transformer of CCVS topologies, it is desired instead by its use in conjunction with the secondary shunt capacitor only to reduce high frequency components, preserving only the direct-current component from the secondary rectifier output.
Summarizing the filter behavior then, the secondary shunt capacitor in the VCCS with full wave rectification class of topologies attenuates high frequency harmonics as it does in the case of the CCVS class of topologies, but unlike in the CCVS class of topologies must work as a filter element in conjunction with the effective primary inductance seen as reflected analytically from the primary to the secondary. The low pass filter creating a direct-current output for the resonant class of VCCS with full wave rectification topologies comprises primary inductance acting as filter inductor, secondary full wave rectifier, and secondary shunt capacitor. The low pass filter creating a direct-current output for the CCVS class of topologies comprises secondary full or half wave rectifier, secondary filter inductor, and secondary capacitor.
It is possible due to resonance to obtain higher working voltages within elements of primary resonant circuits in VCCS topologies than those imposed by the forcing function on the inputs of these circuits. This is a preferable effect to an alternative approach of CCVS topologies that imposes higher working voltage in a secondary circuit, with respect to electrical or electrically related environmental safety of secondary circuits.
An understanding of the VCCS with full wave rectification class of topologies can become slightly more complex in the case of a resonant topology such as one referred in the literature to as an ‘LLC’ wherein a second inductor in shunt with the transformer primary performs an additional role, acting in shunt with the magnetizing inductance of real-world transformers, to promote in the primary circuit greater circulating current than otherwise, which causes the total primary circuit circulating current to exceed that amount necessary for supporting the required reflected load current and magnetizing current of a typical transformer's primary. As this shunt inductor along with the magnetizing inductance of the transformer primary acts in conjunction with the primary series inductance in determining the current being sourced to the transformer primary throughout the entire operational period, in a role supporting resonant behavior of the primary circuit, it forms an integral part of the band-pass filter inductance as described previously.
A resonant inductance that is series electrically connected in the primary circuit becomes a VCCS that acts upon the transformer primary. Behavior of the switches in the primary circuit along with that of the primary capacitor connected in series between the forcing function generated by the switches and the primary series inductance act to create only alternating-current voltage components on the input of the inductor. The differential voltage of the primary series inductance is clamped to the difference between the primary voltage of the transformer, and the sum of forcing function voltage plus the differential voltage of the series electrically connected capacitor. Any direct-current voltage component created by the switches is prevented by behavior of the capacitor from reaching either the primary series inductor or subsequently the transformer's primary. The voltage impressed on the transformer's primary due to the forcing function and primary resonant circuit consequently is alternating-current, and its magnitude is allowed by the voltage-compliance of the primary series inductance to be clamped through the full wave rectifier(s) of its secondary to the reflected magnitude of the power converter's output terminal voltage plus that of reflected magnitudes of forward voltage drops of the rectifier(s) and other secondary series impedances; where voltage polarity of the transformer is in a direction determined by current flowing through it resulting from the changing current levels in the primary series resonant, primary shunt magnetizing, and any primary shunt inductances, so that due to the resulting current and voltage polarities only positive power is input to the transformer primary. Due to voltage-compliance of the transformer's secondary winding(s), and the current-compliance of the secondary circuit connected to it, the transformer output will not impose any voltage magnitude level upon the secondary circuit that exceeds the power converter's output terminal voltage magnitude by more than is necessary to overcome forward voltage drops in the forward biased rectifier and other series impedances of the secondary circuit. The maximum working voltage magnitude in the secondary circuit will therefore be clamped by the secondary rectifier(s) over the full cycle to not exceed the power converter's output terminal voltage magnitude by any significant amount, assuming the output voltage forward drop is significantly greater than the secondary rectifier forward voltage drop(s) plus that due to the influence of series electrically connected secondary impedances, resistances, or other devices.
The VCCS with half wave rectification class of topologies will be mentioned briefly at this point, as it has not been discussed earlier. Without the benefit of full wave rectification, the secondary of the transformer does not have its maximum voltage magnitude clamped to the secondary circuit voltages during the non-conducting half cycle of the rectifier(s). The maximum voltage magnitude of the secondary transformer voltage can rise therefore to its maximum compliance level, imposing this voltage on the input of the non-conducting rectifier(s) installed in the secondary circuit. An intrinsic advantage of limiting secondary maximum voltage magnitudes does not result by using the VCCS with half wave rectification class of topologies. The advantage can be regained in the case where a voltage clamp is employed to ensure the non-conducting half cycle voltage magnitude does not exceed the desired limit.
In the example given of a luminaire, it would be a practical maintenance advantage for the operator to be able to exchange that portion of the system containing only the LED subsystem without being exposed to hazardous potentials, or risk of electric shock. In the example where an operator would have access to an LED subsystem and secondary circuit that it connects to, these would be classified SELV for the purpose of addressing the requirement for electrical safety against electric shock hazard.
Based upon this framework of impedances and filters just developed to understand the relevant behavioral differences between CCVS class, VCCS with half wave rectification class of topologies, and VCCS with full wave rectification class of topologies, two examples are now given for comparison where in order to optimize for secondary circuit efficiency through means of using the highest feasible output voltage for the case of a secondary circuit that is to be classified as SELV, it must be assured through design, either inherently, by active means, or both, that working voltages in the secondary shall not exceed 42.4 Volts alternating-current or 60.0 Volts direct-current under normal conditions or single fault conditions. The alternating-current voltage peak produced by the transformer secondary is the largest magnitude of all voltages present in the secondary circuit of SM topologies considered herein since the maximum direct-current voltage magnitude does not exceed the maximum alternating-current peak voltage magnitude; so the design must limit secondary working voltages to 42.4 Volts alternating-current , which corresponds to a sinusoidal waveform with 60.0 Volts peak, on the transformer secondary for those circuits to be classified SELV.
In a first example one assumes the total forward voltage drop of all series electrically connected elements, exclusive of any inductive element, between the transformer secondary and the power converter output terminals are equal to 2.4 Volts, where an arrangement of secondary full wave current rectification elements is employed that imposes forward voltage drops of two series rectifiers at 1.2 Volts each, and where the topology to be used for power conversion is chosen from the more prevalent CCVS class of topologies; and one further assumes that a minimum duty cycle of 0.3 may occur, signifying the pulse width minimum limit is 30% of the ideal maximum full pulse width, then due to the voltage averaging effect of the CCVS's secondary inductor the load voltage in the ideal case will be the product of the duty cycle, 0.3, and the transformer secondary voltage, 60.0 Volts, minus the secondary circuit's forward voltage drops, 2.4 Volts: 15.6 Volts. This would produce secondary circuit efficiency of the output voltage, 15.6 Volts, divided by the sum of the output voltage, 15.6 Volts, and the forward voltage drops, 2.4 Volts: 0.87. An over-voltage condition could still result in the secondary circuit under a fault condition, as in one case where the mains supply voltage were to be too high, unless the higher working voltage in the secondary can be prevented by an effective fast-acting supplemental protection in the form of a circuit for over-voltage detection that acts to sense and limit secondary working voltage maximum levels.
In a second example, one assumes in the previous example that the topology to be used for power conversion is instead chosen from among the VCCS with full wave rectification class of topologies described herein, and which therefore does not include a secondary inductor capable of performing voltage averaging, and that for the purpose of illustration does include a full bridge rectifier in its secondary, and therefore that the current-compliant secondary circuit does not act by performing a voltage averaging function on the periodic waveshape resulting from the rectified transformer secondary voltage, as in the case of the CCVS class of topologies, then the load voltage will be the transformer secondary peak voltage, 60.0 Volts, minus the total forward voltage drops of 2.4 Volts: 57.6 Volts. This would produce secondary circuit efficiency equal to the output voltage, 57.6 Volts, divided by the sum of the output voltage, 57.6 Volts, and the forward voltage drops, 2.4 Volts: 0.96. This example uses the VCCS with full wave rectification class of topologies to produce secondary circuit efficiency calculated at 9% greater than that calculated in the first example using CCVS class of topologies.
Further, it is necessary in the case of the VCCS with full wave rectification class of topologies as in the CCVS class of topologies, that in order to satisfy criteria ensuring electrical safety of an SELV secondary circuit is maintained in the event of a single fault, there is a requirement that should the control over the primary malfunction, the transformer secondary voltage magnitude must be prevented from becoming higher than planned by design in the given case of an intended 57.6 Volts load voltage. A single fault in this extreme case could result in a voltage excursion of the transformer's secondary that immediately violates the maximum voltage magnitude limit, unless the higher working voltage magnitude can be prevented by an effective fast-acting supplemental form of protection in the form of a circuit for over-voltage detection that acts to sense and limit secondary working voltage levels. In practice, some design margin would be allowed that would accommodate the delay of the protection cicuit by designing for a slightly lower output voltage than the theoretical maximum.
In both the first example of CCVS class of topologies and the second example of VCCS with full wave rectification class of topologies, the criteria for SELV would necessitate an effective protection circuit that acts to sense and limit secondary working voltage levels; but the second example, due to its normally and inherently lower levels of maximum working voltage magnitudes at any given power converter output terminal voltage, continues to offer new utility of higher secondary circuit efficiency while continuing to satisfy criteria for low-voltage classification; or, more generally, having higher secondary circuit efficiency with an advantage of lower working voltage magnitudes in secondary circuits. Even if the required criteria were for ELV classification instead of SELV classification, the second example would continue to offer new utility through higher secondary circuit efficiency resulting from higher load voltage, while imposing lower secondary working voltage maximum magnitudes. By using these two topology examples for both SELV or ELV secondary circuits, one may design a load voltage in the case of the second topology example that is greater than in the case of the first topology example, but less than the maximum ideally achievable as described in the second example, so that an additional advantage resulting from the VCCS with full wave rectification class of topologies over the CCVS class of topologies is that they would have simultaneously lower working voltage magnitudes in the secondary and higher secondary circuit efficiency.
It should be understood that the application example given of an LED luminaire is for illustrative purposes, it being a relevant actual case; and using this example does not imply or intend any limits to the scope of applicability for which exists such requirements for new utility of a class of power converter topologies herein described as being VCCS with full wave rectification. It should be further understood that in the description of how efficiency is to be calculated there should be included a usually small additional current component that conducts through the rectifier but does not conduct through the output load, due to a need to generate internal bias power to satisfy requirements of secondary control circuits. This additional current component has been ignored in the interest of simplicity of illustration, and this simplification does not imply or intend to limit the scope of applicability for which exists such requirements for new utility of a power converter as herein described.
A third example simply recognizes the advantage of lower magnitude of working voltages in secondary circuits, without attempting to satisfy ELV, SELV, or other limited-voltage secondary circuit constraints. A high voltage battery charger system may be incorporated into an electric or a hybrid electric automobile, wherein power converter output terminal voltage would be at relatively high magnitude to properly charge the high voltage battery load. This imposes insulation and isolation requirements upon the design of the power converter charger in order to protect the operator from coming into contact with hazardous voltages. The high voltage power converter is more easily manufactured for a lower cost, and with greater levels of performance and safety assurance, for power converters with isolated secondaries having lower levels of working voltage magnitudes. In this example the VCCS with full wave rectification would have greatly reduced secondary working voltage magnitudes when compared with the CCVS class of topologies. Secondary circuit efficiency may also be improved to a minor degree for the high voltage power converter designed using VCCS, owing to the fact that high voltage rectifiers have typically larger forward voltage drops, and owing to the inability of low forward voltage drop Schottky diodes to be employed, so the primary resulting benefit would be in reducing cost of manufacture and reducing potential safety hazards through reduced insulation and isolation requirements, and the secondary benefit may be in improved secondary efficiency.
Finally, it is observed that a particular topology of a current-fed type along with adoption of appropriate new structural features may be well suited to applications where reduction in package size and weight would represent a benefit, such as for automotive applications, and would be especially well suited to exo-atmospheric applications where insulation and isolation requirements are even more imposing on package size and weight, and where for instance the benefit of reduction in package size and weight would be of high value.